DESCRIPTION: (Applicant's Description) Persistent genomic instability has been observed in the progeny of mammalian cells exposed to ionizing radiation. This instability is manifested as delayed expression of lethal mutations; accumulation of coincidental mutations; and elevated, non-clonal karyotypic heterogeneity. Limited studies with alpha particles have also suggested that high linear energy transfer (LET) radiation is more potent than low LET gamma rays as an inducer of heritable genome instability. The molecular mechanisms underlying radiation-induced heritable genome instability are not known. In the yeast S erevisiae, there is strong evidence that genes of the RAD52 epistasis group, which are essential for DNA double-stand break repair and homologous recombination, are also involved in maintaining the global integrity of the genome. Recently, human homologs of the yeast RAD51 and RAD52 genes (dubbed hhRAD51 and hhRAD52) have been cloned. The hypothesis the applicant proposes to test is that hhRAD51 and hhRAD52 have roles in maintaining genome integrity, such that loss of expression of these genes will exacerbate radiation induced heritable genome instability. The specific aims are: 1. To determine the effects of hhRAD5l/52 deficiency in human cells on ionizing radiation-induced genome instability, 2. To determine whether alpha or HZE particles are more potent than gamma rays as inducers of heritable chromosomal instability in human cells, and 3. To determine whether a temporal relationship exists between heritable chromosomal instability and the expression of a delayed mutator phenotype. In this application, the applicant will focus on the delayed genome instability induced by high LET radiation, particularly high Z, high-energy (HZE) particles. HZE are an important component of galactic cosmic rays and are of particular concern in regard to manned space missions. The applicant will study radiation-induced genome instability using HZE particles of various energies that reflect differences in particle track structure. Systematic analysis of genome changes as a function of radiation parameters will provide a fundamental basis for the effects of high LET radiation on genome stability.